A glycoprotein (IGP) has been isolated from polyvinyl sponge and open wound granulomas that inhibits in vitro phagocytosis by guinea pig peritoneal macrophages. Normal guinea pig tissue extracts or serum do not contain IGP. Extracts of spleen from the autoimmune NZB/NZW mouse, but no other tissues contain inhibitor activity. IGP does not inhibit macrophage migration or resemble MIF. It has no biological effect or immunological identity with the complement system. Antisera to IGP conjugated to fluorescent dye demonstrates by direct immunofluorescent technique that only mononuclear cells in the granuloma react with this reagent. Studies to date indicate that this glycoprotein that contains glucose, galactose, mannose, glucosamine and sialic acid is of local origin in maturing granulation tissue. It may function in vivo as a regulator of macrophage activity. Quantitative analysis of the amino acid content of IGP is in progress and comparative chemical analysis of IGP from young and mature experimental granulomas is now underway. The in vivo activity of IGP on granuloma formation, vascular events, and phagocytosis by the RES will be initiated.